1. Field of the Invention
The present invention relates to a control method for an electric power steering mechanism having a motor for assisting a steering force based on a steering torque generated in a steering shaft and an electric power steering mechanism.
2. Description of the Related Art
The electric power steering mechanism includes a motor for assisting a steering force based on a steering torque generated in the steering shaft and a driving current, obtained by proportional-integrating deviations between a current instruction value determined based on a steering torque, vehicle velocity, rotation angle of this motor and the like and a current detection value flowing actually through that motor, is supplied to this motor. Therefore, if the rotation angle information of the motor obtained from a rotation angle sensor is not normal, a current instruction value determined thereon is not appropriate.
Thus, conventionally, as art for detecting a trouble in the rotation angle sensor which detects the rotation angle of a motor in an electric power steering mechanism and outputs its relative value, for example, an incremental encoder, a method in which an absolute encoder is employed simultaneously and the output pattern (absolute value information) of the absolute encoder is compared with the output pulses (relative value information) of phase A and phase B of an incremental encoder so as to detect a trouble in the incremental encoder, has been well known.
However, because the incremental encoder outputs pulse strings of phase A and phase B with changes in the rotation angle as relative values, when a motor which is a detection object about the rotation angle, is not rotating, no pulse string is outputted from the incremental encoder as a rule, different from the absolute encoder which outputs an absolute value of the rotation angle (about a pulse may be outputted depending on the circuit configuration).
Thus, the configuration for detecting a trouble in the rotation angle sensor (incremental encoder) by using the absolute encoder simultaneously is not capable of detecting the trouble in the rotation angle sensor until the motor, which is a detecting object, is rotated to some extent.
Therefore, if the motor for assisting a steering force of the electric power steering mechanism is not rotated due to some reason, system for detecting a trouble in the rotation angle sensor by using the absolute encoder at the same time cannot detect the trouble in the rotation angle sensor of that motor.
As an example that the motor for assisting the steering force of the electric power steering mechanism is not rotated for some reason, there is a case where the steering wheel is turned ultimately so that a pinion gear of a rack and pinion which construct a steering gear box reaches a rack end (hereinafter referred to as end contact state).
In such an end contact state, even if the pinion rides over the rack, the steering shaft is never rotated further in the same direction. Particularly, if the end contact state is generated by stationary steering, a drive current for generating a torque which blocks a steering wheel turned ultimately from being returned in an opposite direction by a reaction force of a tire is supplied to the motor and controlled so that the reaction force of the tire balances with the steering force. Thus, in the end contact state, it is not necessary to supply so much sufficient drive current to the motor.
However, when the motor is not rotated, the conventional art is incapable of determining which the phenomenon is originated from a trouble in the rotation angle sensor or the end contact state. In such an end contact state, a so sufficient drive current is supplied to the motor thereby leading to generation of heat in the motor.
If the motor is provided with the function for if the temperature of the motor is raised over a predetermined temperature, suppressing the drive current of the motor so as to prevent generation of heat as a function for protecting the motor from the generation of heat, providing a driver""s driving sense with feeling of inconvenience therefore is not preferable.
The present invention has been achieved to solve the above-described problem and therefore an object of the invention is to provide a control method for an electric power steering mechanism capable of detecting an end contact state even if a motor is not rotated and an electric power steering mechanism. Another object of the present invention is to provide a control method for the electric power steering mechanism capable of preventing generation of heat in the end contact state and an electric power steering mechanism.
Still another object of the present invention is to provide a control method for the electric power steering mechanism capable of improving a steering sense when the motor is not rotated and an electric power steering mechanism.
In order to achieve the above objects, a control method for electric power steering mechanism having a motor for assisting a steering force based on a steering torque generated in a steering shaft according to the present invention is characterized in that if it is detected that the condition that the sensor output of a rotation angle sensor which detects a rotation angle of said motor and outputs its relative value is substantially zero and the condition that said steering torque is over a predetermined value are satisfied at the same time, the drive power supplied to said motor is restricted to less than a predetermined value.
A control method for electric power steering mechanism in accordance with the more preferred teaching of the present invention is characterized in that with said drive power restricted to less than the predetermined value, it is determined that if there is a sensor output of said rotation angle sensor, end contact state is present and if there is no sensor output of said rotation angle sensor, motor stop state originated from a trouble in said rotation angle sensor is present.
According to the control method for the electric power steering mechanism of a first aspect and a second aspect of the present invention, if it is detected that the condition that the sensor output of the rotation angle sensor is substantially zero and the condition that the steering torque is over a predetermined value are satisfied at the same time, the drive power supplied to the motor is restricted to less than a predetermined value. If it is detected that both the conditions are satisfied at the same time or the rotation of the motor is stopped, torque generated by the motor is reduced. Thus, the rack shaft is returned in an opposite direction to the rack end by a reaction of a tire if the end contact state is present, so that the pinion gear on that rack is rotated. As a result, the steering shaft coupled with the pinion gear is rotated, so that the motor and the rotation angle sensor can be rotated. Thus, if the end contact state is present, the sensor output can be produced. Therefore, there is such an effect that even if the motor is not rotated, the end contact state can be detected by detecting the sensor output.
Further, because the drive power supplied to the motor is restricted to less than a predetermined value, the drive power supplied more than necessary can be reduced if the end contact state is present. Thus, there is such an effect that generation of heat from the motor under the end contact state is prevented.
Further, because the drive power supplied to the motor is restricted to less than a predetermined value, torque generated by the motor can be reduced if the rotation of the motor is stopped due to a trouble in the rotation angle sensor. Thus, there is such an effect that steering feeling is improved.
An electric power steering mechanism having a motor for assisting a steering force based on a steering torque generated in a steering shaft according to the present invention is characterized by comprising:
a sensor output detecting means for detecting the sensor output of a rotation angle sensor which detects the rotation angle of said motor and outputs its relative value;
a steering torque detecting means for detecting a steering torque generated in said steering shaft;
a state detecting means for detecting such a predetermined state that said sensor output detected by said sensor output detecting means is substantially zero while said steering torque detected by said steering torque detecting means is over a predetermined value; and
a drive power restricting means for if said predetermined state is detected by said state detecting means, restricting the drive power supplied to said motor to less than a predetermined value.
An electric power steering mechanism in accordance with the more preferred teaching of the present invention is characterized by further comprising a state determining means for, with the drive power restricted to less than the predetermined value by said drive force transmitting means, determining that if there is a sensor output of said rotation angle sensor, end contact state is present and if there is no sensor output of said rotation angle sensor, motor stop state originated from a trouble in said rotation angle sensor is present.
In the electric power steering mechanism according to a third aspect and a fourth aspect of the present invention, the sensor output of the rotation angle sensor which detects the rotation angle of a motor and outputs its relative value is detected by the sensor output detecting means and a steering torque generated in a steering shaft is detected by a steering torque detecting means. Then, if a state detecting means detects such a predetermined state that the sensor output detected by the sensor output detecting means is substantially zero and that the steering torque detected by the steering torque detecting means is over a predetermined value, a drive power restricting means restricts the drive power supplied to the motor to less than a predetermined value. Consequently, because the drive power supplied to the motor is restricted to less than a predetermined value, the rack shaft is returned in an opposite direction to the rack end by reaction of a tire, so that the pinion gear on that rack is rotated. As a result, the steering shaft coupled with the pinion gear is rotated, so that the motor and the rotation angle sensor can be rotated. Thus, if the end contact state is present, the sensor output can be produced. Therefore, there is such an effect that even if the motor is not rotated, the end contact state can be detected by detecting the sensor output.
Further, because the drive power supplied to the motor is restricted to less than a predetermined value, the drive power supplied more than necessary can be reduced if the end contact state is present. Thus, there is such an effect that generation of heat from the motor under the end contact state is prevented.
Further, because the drive power supplied to the motor is restricted to less than a predetermined value, torque generated by the motor can be reduced if the rotation of the motor is stopped due to a trouble in the rotation angle sensor. Thus, there is such an effect that steering feeling is improved.